Sealants useful in aerospace and other applications must satisfy demanding mechanical, chemical, and environmental requirements. The sealants can be applied to a variety of surfaces including metal surfaces, primer coatings, intermediate coatings, finished coatings, and aged coatings. In sealants such as those described in U.S. Pat. No. 6,172,179, an amine catalyst is used to provide a cured product. Such systems typically cure in 2 hours to 12 hours and although exhibiting acceptable fuel resistance and thermal resistance for many applications, improved performance of the cured product is desirable.
Michael addition curing chemistries are often used in acrylic-based polymer systems and, as disclosed in U.S. Pat. No. 3,138,573, have been adapted for use in polysulfide compositions. Application of Michael addition curing chemistries to sulfur-containing polymers not only results in cured sealants having faster cure rates and enhanced performance including fuel resistance and thermal resistance, but also provides a sealant with improved physical properties such as elongation. The use of Michael addition curing chemistries for sulfur-containing prepolymer compositions useful in aerospace sealant application is disclosed in U.S. Application Publication No. 2013/0345371, which is incorporated by reference in its entirety.
The compositions disclosed in U.S. Application Publication No. 2013/0345371 employ one or more base catalysts such as amine catalysts. In the presence of an appropriate base such as 1,8-diazabicycloundec-7-ene (DBU), the thiol-Michael addition reaction is fast and the cure time is typically less than 2 hours. Without an appropriate base catalyst, the Michael addition reaction between, for example, a thiol-terminated polythioether and a Michael acceptor reacts slowly, providing a pot life, for example, depending on the temperature, of several days to weeks. The reaction mechanisms for thiol-Michael addition reactions are disclosed by Chan et al., Macromolecules 2010, 43, 6381-6388.
In practice, the foregoing compositions can be provided as two-part compositions in which a thiol-terminated sulfur-containing prepolymer and a Michael acceptor are provided as separate components, with the amine catalyst in one or both components, and the two parts are mixed shortly prior to use. For example, if the catalytic amine is a tertiary amine, the amine catalyst may be in one or both components, and if the catalytic amine is a primary or secondary amine, the amine catalyst can only be included in the component containing the thiol-terminated sulfur-containing prepolymer. Alternatively, the base catalyst may be provided as a third component, and the component containing the thiol-terminated sulfur-containing prepolymer, the component containing the Michael acceptor, and the component containing the base catalyst can be mixed shortly before use. However, once the components are mixed, the Michael addition reaction proceeds, and depending at least in part on the temperature and on the type of amine catalyst, the working time is limited to less than 2 hours. Furthermore, once the composition starts to cure, there is little ability to control the reaction rate to take advantage of the complex chemistries taking place after the sealant is applied to a surface.
Michael addition curing chemistries catalyzed by appropriate bases such as primary or secondary amines are used in aerospace sealants. For example, Michael acceptor-terminated prepolymers suitable for use in aerospace sealant applications are disclosed in entitled U.S. application Ser. No. 14/200,569, filed on Mar. 7, 2014, U.S. Application Publication No. 2014/0378649, and U.S. Application Publication No. 2015/0119549, each of which is incorporated by reference in its entirety. Sulfur-containing prepolymers terminated with Michael acceptors such as vinyl sulfones or maleimides react rapidly with polythiols at room temperature in the presence of primary or secondary amine catalysts.
Tertiary phosphines are known to catalyze Michael addition reactions. Michael addition curing of coating compositions using phosphine catalysts is disclosed, for example, in U.S. Application Publication No. 2010/0068393. Phosphine catalysts provide rapid curing within a few seconds even at room temperature and therefore can be useful in spray coat applications.